League of Legends has a lot of spaghetti code and some of it is preventing cooldown reduction from working in certain situations.
Cooldown reduction has always been a crucial part of a large portion of champion builds in League of Legends . You just can't imagine any caster, be it AD or AP, without striving for as much CDR as you can get. Even after the change to Ability Haste, people just can't get enough of it and yet, most of us weren't aware that in some situations we were getting none of it, for at least the past six years.
This interesting and highly useful bit of info is coming from the League of Poro channel on YouTube as the player's testing concluded that you don't get the effects of Ability Haste for a short bit of time after respawning at the fountain. In other words, if you're spamming an ability in order to cast it as soon as you're up, it will go on full base cooldown, without factoring in any Ability Haste you may have.
Therefore, a Karthus ult will still go down for three minutes even if you built over 100 AH. He is probably the most frequent victim of this bug as a Requiem can often secure more than one kill towards the end of a fight and as such, you want to get it off as soon as you respawn.
It seems like this bug was around for more than half a decade and unfortunately, Riot never fixed it.
